Switch to CodecZlib from GZip

[alyst]

I'm not sure whether writetable() is already at its end-of-life stage, but if it stays for a while, it might be better to switch to CodecZlib.jl from GZip.jl as the latter is deprecated and not actively maintained.

[coveralls]

Coverage remained the same at 72.926% when pulling 4b7cb02 on alyst:use_codeczlib into bb1e8d6 on JuliaData:master.

[nalimilan]

I'd have said it has reached its end of life since it's deprecated, but since the code is ready...

[cjprybol]

Thanks @alyst! I agree with @nalimilan here that we probably don't want to fix anything related to readtable and writetable going forward now that they're deprecated, but you already have the PR ready to go so why not take it.

[cjprybol]

Have we set an end-of-life date for read/writetable functions? DataFrames 0.12? Julia 1.0? Maybe each version release (0.11.2, 0.11.3, etc.) we add a sleep(10) call until the delay becomes so unbearable that everyone voluntary converts their code to use CSV.jl?

[nalimilan]

I don't think we're in a hurry to remove them, they're not a big burden. It would be logical to remove them in the next pseudo-major release, so 0.12 I guess.